This invention relates to an electronic copier or printer of the type which uses a raster output scanner (ROS) to form images on a moving photoreceptor surface. More particularly, the invention is directed towards a spatial filter placed along the optical path for compensating for errors in imagined scan lines due to mechanical vibration present in the photoreceptor.
In document reproduction systems which incorporate scanning illumination systems (moving illumination sources, reflectors, projection lens) to project an image onto a moving photoreceptor, it is known that undesirable exposure strobing can result from mechanical vibrations of the various components comprising the optical system.
U.S. Pat. No. 4,699,497 describes a method and apparatus for compensating for the effects of vibration of a scanning illumination lamp as it accelerates through a scan cycle. The compensation for this system relies on increasing the natural frequency of the illumination lamp to nullify the effects of the lamp vibration.
Another common cause of strobing in prior art light lens systems is the non-uniform (vibrational) motion of the photoreceptor, whether of the drum or belt type. In copending application U.S. Ser. No. 910,708 assigned to the same assignee as the present invention, there is disclosed an optical transmission filter which is placed in the optical path. The filter has specified transmittance characteristics which result in reduction or elimination of the strobing.
Mechanical vibration of the photoreceptor is also a problem in non-light lens imaging systems. Examples of printers using image bars to print on the surface of a photoreceptor are found in U.S. Pat. No. 4,477,175, where the image bar is composed of a plurality of individually activated illumination elements (LEDs), and in U.S. Pat. No. 4,389,659 where the image bar is composed of a plurality of individually activated electro-optic electrodes. An example of a system using a raster output scan (ROS) optical system is disclosed in U.S. Pat. No. 3,867,571. In this patent, a laser beam is modulated in accordance with an image data input and an image reflected from a rotating polygon to form a scan line at a photoreceptor surface. For this type of printing system, photoreceptor vibration can cause periodic modulation in the gray regions, a phenomenon known as banding. Periodic exposure modulation in raster-generated images is the digital analog of exposure strobing caused by mechanical vibrations in light-lens scanning-type copiers. Resultant output copies are subject to solid area exposure non-uniformity banding in half tones and variations in line width.
The present invention is directed towards a system for compensating for periodic exposure modulation in images transmitted by a raster output scan device to the surface of a photoreceptor, the photoreceptor being subject to mechanical vibration. More specifically, the invention is directed towards a laser printing system comprising means for generating a beam of high intensity light, a moving recording medium sensitive to said high intensity light, optical means comprising a plurality of optical elements interposed between said light generating means and said recording medium, and including a cylindrical lens positioned in proximity to said recording media for imaging said beam to a spot at the surface of said medium, modulating means located between said light generating means and a scanner device for modulating the light beam in accordance with the information content of electrical signals, means coupled to said modulating means for deflecting said modulated light beam to a predetermined position on said scanner, said scanner device comprising a multi-faceted polygon located in the path of said modulated beam between said beam generating means and said recording medium and having reflective facets for reflecting the beam incident thereon onto said medium, means for rotating said polygon such that the reflected light is scanned in successive traces in any direction across said medium, said system further including an amplitude transmittance spatial filter positioned in the first Fourier plane of said cylindrical lens.
The following references contain disclosures which appear to be material to the present invention.
U.S. Pat. No. 4,305,650 to Knox discloses an illumination system which uses a transmittance filter for eliminating strobing. The transmittance filter overlays a plate defining an aperture between that filter and a photoconductive member.
U.S. Pat. No. 4,153,336 to Minami et al discloses an apparatus for detecting defects of a pattern using a spatial filter located on the Fourier transform plane of a Fourier transformed informtion light.
U.S. Pat. No. 4,647,154 to Birnback et al discloses an optical image processor based on geometric and diffraction optical systems for feature enhancement, wherein Fourier processing and spatial filtering is employed.
A copending application U.S. Ser. No. 139,391 assigned to the same assignee as the present invention discloses a method to compensate for effects of photoconductor vibration in a printing system utilizing an image write bar. Photoconductor vibration is detected and electrical signals are generated which are used to adjust the on/off timing of the write bar operation and the intensity of the image bar output. These adjustments result in compensation for the effects of photoconductor vibration.